当 socket 的另一端发送一个 FIN 包的时候触发，从而结束 socket 的可读端。
Emitted when the other end of the socket sends a FIN packet, thus ending the readable side of the socket.
By default (
false) the socket will send a FIN packet
back and destroy its file descriptor once it has written out its pending
write queue. However, if
allowHalfOpen is set to
true, the socket will
end() its writable side, allowing the
user to write arbitrary amounts of data. The user must call
end() explicitly to close the connection (i.e. sending a
FIN packet back).